This invention relates to high frequency electrical networks having frequency dependent coupling properties so that signals at one frequency can be combined with, or separated from, signals at another frequency while maintaining electrical isolation between respective signal sources or loads as the case may be. One requirement of this kind arises in the combination of broadcast signals having different carrier frequencies generated by different transmitters so that they can be radiated at a common antenna, but without the output of one transmitter coupling into or adversely affecting another transmitter.
According to this invention a high frequency network includes a transmission line device in the form of a closed cavity having two opposite conductive end plates and a connecting side wall structure; four quarter wave resonators mounted within the cavity and being disposed symmetrically about an axis passing through both end plates, one pair of mutually opposite resonators being mounted on one end plate, and the other pair being mounted on the other end plate; and a coupling loop being mounted on the side wall structure so that it couples equally with the two closest resonators.
Preferably each resonator is in the form of a hollow tube which is closed at the end which is mounted on the end plate, and is open at its other end which is spaced apart from the opposite end plate.
Preferably again the open end of each tubular resonator is capacitively coupled to its said opposite end plate by conductive means which project into the interior of the open end.
Coupling means additional to said coupling loop are generally provided, with the additional coupling means also being positioned so that it couples equally into two of the resonators. The additional coupling means may be another coupling loop, in which case it may couple into a different pair of resonators, or into the same pair of resonators in which latter case it will be displaced longitudinally with respect to said axis so that both loops are accommodated on a common longitudinal line. Alternatively, the additional coupling means may serve to provide coupling to a further transmission line device similar to the first mentioned device. By cascading two or more similar transmission line devices the frequency response of the network can be modified to meet particular requirements.
The network is very compact and simple to construct as compared with previously known networks using discrete cavities linked by external hybrid circuits and transmission lines. It can be implemented very satisfactorily for frequencies of the order of 100 MHz and for frequencies of this order it occupies significantly less space than the network disclosed in our previous United Kingdom Pat. No. 1,390,809 (corresponding to U.S. Pat. No. 3,886,499).